Complex Patterns Complex Patterns of Inheritanceof Inheritance

Exceptions to Mendel’s rules:Exceptions to Mendel’s rules:

not simple dominant/recessive inheritancenot simple dominant/recessive inheritance

Incomplete DominanceThe phenotype of the heterozygote is

INTERMEDIATE.

Red + White = Pink

RR’ RR’

RR’ RR’

R R

R’R’

R’R’

Red parent plant

White White parent parent plantplant

All pink offspring!

CCooddoommiinnaanncceeBoth alleles are expressed!

Red + White =

Both Red and White (NOT pink!)

Pleiotropy

The ability of a gene to affect an organism in many ways, not just in one way.

• Example: Human disease PKU (phenylketonuria)

• Causes mental retardation and reduced skin & hair pigmentation

• Caused by mutation in gene for creating the enzyme that breaks down the amino acid phenylalanine

• This mutation makes the person unable to break down phenylalanine, leading to toxic levels that can damage the body in many ways.

• PKU is manageable with medications and by following a diet free of phenylalanine.

PKU patient

Epistasis

A gene at one location affects the expression of

a gene at another location.

Coat color in mice:BB & Bb (black) bb (brown)

Gene that allows color to be deposited:

CC & Cc (color is present)cc (albino)

Multiple Alleles and Codominance

This is a gene that is controlled by more than two forms of an allele

• Blood Types: A, B, AB, O

• A and B are codominant

• O is recessive to both

Type A: IAIA or IAi Type B: IBIB or IBi

Type AB: IAIB Type O: ii

Polygenic Traits

Trait controlled by two or more GENES

(not alleles)

AaBbCc x AaBbCc

• Skin Pigmentation• Height

Sex-Linked TraitsHumans have 46 chromosomes

22 pairs autosomes (pairs 1-22)1 pair sex chromosomes (pair #23)

Males: XY Females: XX

Sex-Linked traits are located on the X chromosome. These traits tend to affect males more than

females. WHY??

Baldness

Colorblindness is a sex-linked trait.

People with normal vision should see a light image of a red/brown sailboat.

Epigenetics